Upload the plugin files to /wp-content/plugins. Don't forget to upload the main folder as well, so that everything goes into /wp-content/plugins/pixelpost-widget. Download the pixelpost.php file from the includes folder of your pixelpost installation and upload it into /wp-content/plugins/pixelpost-widget/includes. It'll overwrite the sample file I've included with the plugin, if it doesn't overwrite it then you may not be uploading it into the right place! It's this file that tells the plugin how to connect to your pixelpost database to retrieve the images, the plugin won't work without it.

Activate the plugin through the 'Plugins' menu in WordPress

Look at the "Pixelpost Widget" section in the "settings" tab on the admin menu, and set the display options there